From the World Heritage inscription:
The Stonehenge, Avebury, and Associated Sites World Heritage property is internationally important for its complexes of outstanding prehistoric monuments.
It comprises two areas of chalkland in Southern Britain within which complexes of Neolithic and Bronze Age ceremonial and funerary monuments and associated sites were built. Each area contains a focal stone circle and henge and many other major monuments. At Stonehenge these include the Avenue, the Cursuses, Durrington Walls, Woodhenge, and the densest concentration of burial mounds in Britain. At Avebury, they include Windmill Hill, the West Kennet Long Barrow, the Sanctuary, Silbury Hill, the West Kennet and Beckhampton Avenues, the West Kennet Palisaded Enclosures, and important barrows.
Stonehenge is unquestionably one of the most recognizable landmarks in the world. There are few people who can’t immediately recognize it from a photo. It is often placed on lists of world wonders and was even commemorated in song by Spinal Tap.
The actual site of Stonehenge wasn’t quite what I expected. There is a road which passes surprisingly close to it, but you never actually see it in photos. Also, the impression of large megalithic blocks is somewhat toned down by the fact that you can’t actually get close to the stones anymore. You can only walk around the structure at a distance.
As I write this in December 2012 they are in the process of replacing the current visitor center and removing the road which passes by. By 2014 the entire area should be green pasture with the visitor center about a mile away, out of sight.
Stonehenge can easily be visited by day trip from London. You can often find trips that will take you there for £30, not including admission.
View my complete list of UNESCO World Heritage sites.